1. Field of the Invention
The present invention concerns a method of and an installation for separating solids and liquids (in particular, but not exclusively, for clarifying or purifying effluent) by coagulation, flocculation, and sedimentation without separator plates (i.e. simple sedimentation).
2. Description of the Prior Art
This invention is in the tradition of techniques for treating effluent by physical-chemical treatment (with formation of floc) followed by sedimentation (referred to as physical-chemical sedimentation for short).
The first simple physical-chemical sedimentation devices allowed operation at a "mirror" rate (i.e. the flowrate treated in M.sup.3 /h per unit surface area of the sedimentation device in M.sup.2) on the order of one meter per hour; this upper limit was imposed by the lightness of the physical-chemical floc and the low settling speed of the floc in effluent.
French Patent 1,411,792 discloses a major improvement consisting in the simultaneous addition to the effluent to be treated of auxiliary solid clarification agents (for example fine sand with a particle size from 20 .mu.m to 200 .mu.m), a polymer and optionally other chemical products routinely used in purification of effluent (flocculation agents).
The floc that forms traps grains of sand which increase its density and therefore its settling speed; the effluent charged with floc flows into a sedimentation reactor from which it overflows (whence a decreasing field of speed from the entry at the bottom to the exit at the top); settling speeds in the order of 6 m/h to 8 m/h are routinely achieved in installations using this principle (these installations are usually called "CYCLOFLOC" installations).
The sand is recovered, for example by hydrocycloning of the sandy sludge, and recycled to the entry of the installation.
French Patents 2,553,082 and 2,071,027 describe a technique in which contact between the untreated effluent and the granular material is brought about by upward flow of the untreated effluent through a fluidized bed of the granular material (sand, in practice), with an upward speed that decreases continuously, and with the concentration regulated with the aid of a sample taken in the fluidized bed of a purge of sand laden with sludge which is recycled after separating the sludge. Separator plates are provided to improve sedimentation above the fluidized bed.
The benefits of using inclined separator plates for treatment of effluent is discussed in many documents including U.S. Pat. Nos. 4,142,970; 4,290,898 and 4,388,195.
"FLUORAPID" sedimentation units using the above principle can achieve mirror speeds up to 8 m/h to 15 m/h.
Another sedimentation method is described in French Patent 2,552,082; this method does not use sand; instead a thickening and sedimentation intermediate chamber is provided between a reaction chamber (flocculation and/or precipitation) and a sedimentation chamber with separator plates. The reaction chamber has two chambers in communication at their top and bottom ends and an axial flow screw causing a strong flow between the two chambers. The reaction chamber also receives some of the sludge recovered from the bottom of the intermediate chamber.
The effluent charged with floc overflows into the upper part of the intermediate chamber in which the floc is thickened and in which 85% to 90% of the floc is deposited on the bottom of the chamber. The partially clarified effluent then enters a sedimentation chamber with separator plates for further sedimentation. It seems that speeds in the order of 35 m/h can be obtained in this way if the constraints in terms of treated effluent quality are moderate.
A more recent proposal is a treatment method also directed to increasing the sedimentation outlet speed without compromising the quality of the treated effluent and is described in U.S. Pat. No. 4,927,543; this method is implemented using installations usually called "ACTIFLO" installations.
In this method the effluent is injected into a stirred chamber for mixing and destabilizing the colloids into which the granular material (fine sand, in practice) and the reagents are also fed. The mixture of effluent, sand, reagent and floc being formed is then passed into a stirred intermediate aggregation chamber in which the floc that began to form in the mixing chamber increases in size without settling. The mixture of effluent and aggregated floc is then fed into a sedimentation chamber with separator plates. The mixture of sand and sludge recovered from the bottom of the sedimentation chamber with separator plates is hydrocycloned and the sand is recycled to the mixing chamber.
This method produces effluent of excellent quality with mirror settling speeds up to around 100 meters per hour.
It is clear that the successive advances resulting from the techniques mentioned above consist, for a given quality of treated effluent, in an increase in the settling speed, firstly using ballasted floc (CYCLOFLOC), then by combining the use of sedimentation using separator plates with floc ballasted with sand (FLUORAPID) or sludge that has previously settled, and finally by improving the quality of preparation of the floc through use of specific stirring conditions, again in combination with sedimentation using separator plates (ACTIFLO).
Thus all the recent methods described in the literature, including the ACTIFLO method, are based on the use of separator plates for sedimentation wherever mirror settling speeds in excess of 15 m/h, for example, are required.
These separator plates represent a non-negligible element of the cost of the installation, both through their inherent cost and through the resulting installation and cleaning constraints.
The present invention is therefore directed to improving the overall economy of sedimentation by eliminating the separator plates without compromising the quality of the treated effluent.
In a manner that the person skilled in the art will find surprising and unexpected, we have discovered that careful physical-chemical preparation of the floc around a granular ballasting material of the type proposed in U.S. Pat. No. 4,927,543 can yield high settling speeds despite the absence of separator plates and therefore results in a process that is more economical and simpler than existing processes.
As explained below, the floc formed by the method of the invention yields mirror speeds of as much as several tens of meters per hour, which is very much greater than the speeds of at most around ten meters per hour obtained with flocs produced using the CYCLOFLOC method, combined with treated effluent of very high quality.